1. Field of the Invention
The present invention relates to an optical head device for use in recording information on an optical information medium and reproducing information recorded on the optical information medium, an optical information apparatus (including recording/reproducing apparatuses and read-only apparatuses) using the same and a system in which they are used.
2. Description of the Related Art
In recent years, in-depth research has been conducted to achieve higher density for optical disk systems by increasing the numerical aperture (NA) of an objective lens to decrease the diameter of a focus spot on an optical disk. For example, the NA of an objective lens of a CD system is 0.4, and the NA of an objective lens of a DVD system is 0.6, whereas the NA of an objective lens of a next generation disk system will be as large as 0.85. In this case, the in-plane distribution of the light incident on the aperture of the objective lens is an issue.
This can be explained as follows. If the in-plane distribution of the light incident on the aperture of the objective lens is constant, the diameter of a focus spot that is focused on an optical disk by the objective lens is represented by λ/NA, where λ is a wavelength of a light source. In addition, the relationship of NA=r/f is satisfied, where r is the aperture radius of the objective lens, and f is the focal distance of the objective lens.
The aperture radius r and the focal distance f are generally determined by the physical size of the objective lens, but it is easily inferred that, for example, the effective aperture radius when the light amount is 0 at the periphery of the aperture is smaller than the physical aperture radius of the objective lens. Therefore, even if efforts are made to achieve higher density of optical disk systems by increasing the NA of the objective lens, that is, even if efforts are made to decrease the diameter of the focus spot on the optical disk, higher density cannot be achieved without the in-plane distribution of the light incident on the aperture of the objective lens being as uniform as possible.
The non-uniform in-plane distribution of the light incident on the aperture of the objective lens conventionally has been a problem. This is caused by the fact that the light intensity of the laser beam emitted from a semiconductor laser light source is not uniform in the light flux.
This problem will be described with reference to FIGS. 16 and 17. FIG. 16 is a view showing the relationship between a laser beam emitted from a semiconductor laser light source and the amount of light captured by a collimator lens, and FIG. 17 is a diagram showing light intensity distribution with respect to an angle at which a laser beam emitted from the semiconductor laser light source diverges (hereinafter, referred to as “diverging angle”). As seen from FIG. 17, the light intensity of a laser beam emitted from the semiconductor laser light source 10 decreases in the manner of a Gaussian function as the light flux radius from the center of the collimator lens 9 increases. Therefore, in prior art, in order to make the intensity distribution in the collimated light flux 8 entering the aperture radius of the objective lens as uniform as possible, the radius rc and the focal distance fc of the collimator lens 9 are adjusted (i.e., the relationship: the capturing NA of the collimator lens 9=rc/fc is adjusted). Thus, only the laser beam within the angle θd of the laser beam emitted from the semiconductor laser light source 10 is captured by the aperture of the objective lens.
Naturally, the smaller the capturing NA of the collimator lens 9 is, the less the intensity distribution in the collimated light flux 8 is, but the utilization efficiency of the laser beam emitted from the semiconductor laser light source is reduced. Therefore, the capturing NA of the collimator lens 9 is determined in view of the balance between the intensity distribution in the collimated light flux 8 and the utilization efficiency of the laser beam. In general, this value is set to about 0.2. As described above, in the next generation optical disk systems, the numerical aperture NA of the objective lens is as large as 0.85 to achieve a higher density than that of DVD systems, and a semiconductor laser light source in a wavelength of 405 nm is used.
On the other hand, for glass material for lens production, as the wavelength of a light source becomes shorter, a change in refractive index with respect to a change in wavelength becomes larger. In general, a change in refractive index of the glass material used for a lens when a wavelength of a light source is changed 1 nm in the vicinity of 405 nm is about three to four times larger than that at a wavelength in the vicinity of 650 nm for DVD reproduction.
When the temperature of a semiconductor laser light source having a wavelength of 405 nm is changed, the emission wavelength is varied, so that the refractive index of an objective lens is changed. Thus, the refractive index of the objective lens is displaced from the refractive index at the time of design, so that the shift amount of the focus spot from the surface of the optical disk by the objective lens is about three to four times larger than that for DVD (chromatic aberration of the objective lens). Furthermore, the lens refracts beams more strongly in a portion closer to the perimeter, so that the beams that pass through the portion closer to the perimeter of the objective lens are affected more by the change in the refractive index. Therefore, a larger focus shift due to the chromatic aberration of the objective lens occurs for the beams that pass through the portion closer to the perimeter of the objective lens, and substantially no focus shift occurs for paraxial beams.
On the other hand, when the NA of the objective lens is increased for higher density, the depth of focus is reduced in inverse proportion to the square of the NA. Therefore, the depth of focus of a system having an NA of 0.85 is only ½ of the focus depth of a system having an NA of 0.6.
Therefore, the focus shift due to chromatic aberration in a next generation disk system (NA of 0.85, a wavelength of a light source of 405 nm) is eight times more demanding than that of a DVD system. Therefore, in the next generation optical disk system, it is necessary to pay attention to the shift of the focus position due to a variation of the wavelength of the light source. When it takes 10 msec or more for the focus position to shift, the focus shift is detected by a focus error detection method, and the objective lens is moved accordingly so as to cancel this focus shift. Therefore, the shift of the focus position due to variations in the wavelength of the light source is not a problem. However, when it takes 10 msec or less for the focus position to shift, for example, the focus is displaced at the time of switching of recording/reproduction of the semiconductor laser light source, which results in poor recording/reproduction and causes a large problem.
As shown in FIG. 18, in order to reduce the chromatic aberration, the objective lens 1 includes three lenses 1c, 1f and 1e that form two groups. The lens 1c is a convex lens and the lens 1f is a concave lens, so that when the emission wavelength of the semiconductor laser light source is shorter than a central wavelength of 405 nm, the refractive index of the glass material constituting the convex lens is slightly increased. Therefore, the convex lenses 2b, 1c and 1e refract beams strongly, so that a focus spot 4 that is focused on a signal surface of an optical disk 3 is shifted to the side of the lens 1e. On the other hand, when the emission wavelength of the semiconductor laser light source is longer than a central wavelength of 405 nm, the refractive index of the glass material constituting the convex lens is decreased. Therefore, the refraction of beams by the convex lenses 2b, 1c and 1e becomes weak, so that the focus spot 4 that is focused on a signal surface of the optical disk 3 is shifted to the side opposite to the lens 1e. 
On the other hand, concave lenses 2a and 1f act on the beams in the opposite manner to the convex lenses 2b, 1c and 1e. Therefore, when the emission wavelength of the semiconductor laser light source is varied, a change of the beams by the convex lenses 2b, 1c and 1e is cancelled by the change by the concave lenses 2a and 1f, so that the shift of the focus spot 4 can be suppressed. The shift amount of the focus spot 4 due to variations of the emission wavelength of this semiconductor laser light source is larger as the curvature of the spherical surface of the lens is larger. Therefore, the shift of the focus spot 4 by the convex lenses 2b, 1c and 1e is mostly cancelled by the concave lens 1f that has a large curvature. Thus, when the objective lens 1 is constituted by three lenses 1c, 1f and 1e forming two groups in this manner, even 1f the emission wavelength of the semiconductor laser light source is changed 1 nm from 405 nm, the shift amount of the focus spot 4 from the signal surface of the optical disk 3 can be restricted to be about 0.001 μm. However, in the case of this lens arrangement, two more lenses are required than when a single lens is used as the objective lens 1 for CD systems and DVD systems, so that an adjusting process becomes complicated. Furthermore, when the objective lens 1 is constituted by a single lens as shown in FIG. 17, simplification of an assembly process and a reduction of the number of lenses reduce costs. However, the shift amount of the focus spot 4 due to the chromatic aberration is as much as 0.5 μm. Therefore, it is necessary to add some element to reduce the chromatic aberration in this case.
In the optical head device shown in FIG. 20, an objective lens 1 constituted by two lenses is used to reduce costs. In this arrangement, not only are the costs reduced, but also the chromatic aberration can be reduced more than in the case of the objective lens 1 constituted by a single lens. Nevertheless, the shift amount of the focus spot 4 due to the chromatic aberration is about 0.35 μm, and it is necessary to add some element to reduce the chromatic aberration in this case as well.
When the objective lens 1 as shown in FIGS. 19 and 20 is used, a chromatic aberration correction element 7 constituted by a diffraction grating is inserted in order to reduce the chromatic aberration that occurs at the time of a variation of the emission wavelength of a semiconductor laser light source. In this case, compared to the objective lens 1 having three lenses in two groups shown in FIG. 18, one or two lenses are eliminated and the chromatic aberration correction element 7 is added. However, since this chromatic aberration correction element 7 can be formed in a simple manner by utilizing one surface of the convex lens 2b constituting a beam expander 2 when the convex lens 2b is formed with resin, the costs can be reduced significantly, compared to the case where the objective lens 1 having three lenses in two groups shown in FIG. 18 is used.
This approach of reducing the chromatic aberration has been well-known for a long time (e.g., JP 2001-60336A, which is referred to as “first conventional example”), but when the amount of the chromatic aberration of the objective lens 1 increases, the grating pitch of the chromatic aberration correction element 7 decreases.
This chromatic aberration correction element 7 can reduce the chromatic aberration for the following reasons. As described above, for example, when the emission wavelength of the semiconductor leaser light source is shorter than a central wavelength of 405 nm, the refractive index of the glass material constituting the convex lens increases and the power of the convex lens increases. Therefore, beams are refracted strongly and the focal distance becomes shorter. On the other hand, the relationship between the wavelength λ and the angle of diffraction θh at the diffraction grating constituting the chromatic aberration correction element 7 is θh=λ/p, where p is the grating pitch of the diffraction grating, and therefore when the wavelength becomes shorter, the angle of diffraction becomes smaller. Therefore, the chromatic aberration correction element 7 acts on beams in the opposite manner to the convex lens. Thus, it is possible to cancel the focus shift caused by the objective lens 1 due to wavelength variation by inserting such a chromatic aberration correction element 7. In this case, since the dependence of the diffraction angle on the wavelength is utilized, the larger the amount of chromatic aberration to be corrected is, the larger the angle of diffraction θh with respect to the wavelength variation has to be. Therefore, when the chromatic aberration of the objective lens 1 becomes larger, the grating pitch of the chromatic aberration correction element 7 becomes narrower, and the grating pitch of the chromatic aberration correction element 7 becomes rougher in a portion closer to the paraxial at the inner circumference.
As described above, the shift amount of the focus spot 4 due to the chromatic aberration in the case where the objective lens 1 having two lenses is about 0.35 μm, and the grating pitch of the collimator lens 9 to cancel this chromatic aberration is about 6 μm at the outermost portion of the effective diameter, and about 150 μm at the central portion. Thus, when the grating pitch is changed significantly, the diffraction efficiency in each radius position of the chromatic aberration correction element 7 is changed as shown by the solid line in FIG. 2A. Therefore, beams in the vicinity of the center of the objective lens 1 are achromatized by the diffraction grating having a pitch of 150 μm, and therefore the diffraction efficiency in this portion is 99%. On the other hand, beams in the outermost portion of the effective diameter of the objective lens 1 are achromatized by the diffraction grating having a pitch of 6.5 μm, and therefore the diffraction efficiency in this portion is about 92% (the diffraction efficiency with respect to the pitch is a value as a result of taking an estimated reduction amount due to processing error that can occur in practical use into account, based on the theoretical value).
Next, as a second conventional example, an arrangement disclosed in JP7-262594A will be described with reference to FIG. 21. In FIG. 21, reference numeral 41 denotes an optical disk, and reference numeral 42 denotes a semiconductor laser light source. Reference numeral 43 denotes a hologram that splits diffracted light 431 in a direction oblique to the optical axis of the incident beam in such a manner that the diffracted light does not enter other optical elements. The laser beam that is emitted from the semiconductor laser light source 42 and enters the hologram 43 is diffracted so as to be converted to light beams having a constant light intensity in the vicinity of the center and passes through the hologram 43 (zero-order diffraction). The upper surface of the grating constituting the surface of the hologram forms a smooth curve. Reference numeral 45 denotes an objective lens for focusing the light beam having a constant light intensity in the vicinity of the center that has passed through the hologram 43 on the optical disk 41 to form a focus spot. Since the light beams are made to have a constant light intensity in the vicinity of the center by the diffraction of the hologram 43, the light beams are focused by the objective lens 45 such that the diameter of the focus spot formed on the optical disk 41 can be a small spot in which the 1/e2 width is substantially equal to 0.96λ/NA.
In the chromatic aberration correction element 7 for correcting the chromatic aberration occurring in the objective lens 1, the grating pitch becomes smaller toward the perimeter, and the diffraction efficiency is reduced toward the perimeter. Therefore, the light intensity in the vicinity of the perimeter of the objective lens 1 is reduced significantly, corresponding to a reduction in the manner of a Gaussian function of the intensity of the semiconductor laser light source with respect to the radius distance of the light flux.
When the light intensity in the vicinity of the perimeter of the objective lens is reduced significantly, the effective NA of the objective lens is reduced. As a result, light cannot be focused sufficiently on the optical disk, and the recording density on the optical disk cannot be increased in proportion to the NA.
Furthermore, in the second conventional example, the angle of diffraction should be large so that the diffracted light 431 is split in a direction oblique to the optical axis of the incident beam in such a manner that the diffracted light does not enter other optical elements. As a result, the grating pitch of the hologram 43 is as small as 2 μm or less, and this is difficult to produce. In addition, the light intensity in the vicinity of the center is constant. Furthermore, the light beams 421 emitted from the semiconductor laser light source 42 constitute a so-called Gaussian distribution in which the intensity in the center is strongest, and the light amount decreases gradually as approaching the perimeter. Therefore, the diffraction efficiency of the hologram 43 should be highest in the center, that is, the zero-order transmittance should be low, and the diffraction efficiency should become lower gradually, that is, the zero-order transmittance should become higher, as approaching the perimeter. Thus, the diffraction efficiency of the hologram 43 is changed depending on the portion, so that if there is a displacement with the center of the light intensity of the light beams 421, the light amount distribution of zero-order transmitted light is changed significantly, which makes it difficult to form a small focus spot as desired on the optical disk.